1. Field of the Invention
The present invention relates generally to devices for guiding drill bits at a predetermined location and angle to a work piece for drilling holes therein, and relates more particularly to a drilling jig for stair rails for drilling the spindle holes in the underside thereof at the correct location and angle.
2. Background of the Invention
The installation of stair rails is an intricate and time consuming process which normally requires the skills of an experienced finish carpenter. Although the rail, supporting spindles and end posts are conventionally fabricated away from the construction site, the spindle holes in the underside of the rail must be drilled at the construction site to ensure the perpendicularity of the spindles. It is, in fact, usually necessary to install the rail in position with the lower end mounted on the knule post and the upper end supported either by a wall bracket or post as called for by the particular installation before the spindle hole positions and angles can be established.
By locating the exact position for the lower ends of the spindles on the stairs, the proper location for the spindle holes in the underside of the rail can then be determined. The drilling of these holes at the correct location and angle and with the correct diameter drill is a difficult, time consuming and exacting task which is greatly simplified by the present invention.
In the typical staircase, there are two spindles for each step, although occasionally three, and rarely four spindles may be based on each step. The upper ends of the spindles are tapered, and since the exposed lengths of the spindles on each step will differ due to the inclination of the stair rail, the holes to be drilled in the stair rail above each step will be of different diameters corresponding to the diameter of the tapered portion of each spindle at its intersection with the lower rail surface.
In my U.S. Pat. No. 4,793,747, issued Dec. 27, 1988, a stair spindle jig is disclosed which simplifies the drilling of stair rail spindle holes. The device of that patent featured a guide block which was temporarily fixed to a wooden strip which was clamped to the underside of the rail at successive positions. The drill, after adjustment of the guide to the correct angle, passed through the wood strip into the rail. The wood strip was discarded and replaced for each rail, extending the amount of time required to set up the jig for each job. Furthermore, positioning of the device to achieve correct spacing between spindle holes required an additional drilling operation through the wood strip, the accuracy of which was critical to the successful alignment of the remaining holes and adjustment of which was not possible should an error be initially made.
A further disadvantage of my earlier patented device was the need for drill guides comparable in diameter to the drills being employed. In addition, the drill guides were subject to wear due to the direct impingement of the drill cutting edges against the guides, leading to a short guide life.